Protectors for electric circuits



Get. 17, 1967 A. LJRAN! 3,34%,Ufl?

PROTECTORS FOR ELECTRIC CIRCUITS Filed Nov. 7, 1966 FIG.8.

United States Patent 3,348,007 PROTECTORS FOR ELECTRIC CIRCUITS Angelo Urani, St. Louis, Mo., assignor to McGraw- Edison Company, Elgin, L, a corporation of Delaware Filed Nov. 7, 1966, Ser. No. 592,423 12 Claims. (Cl. 200-131) ABSTRACT OF THE DISCLOSURE A bridge structure for an electric fuse has spaced-apart conductors thereon, the confronting faces of those conductors are generally straight and are inclined to the axis of the housing for that electric fuse, those conductors have sections adjacent those confronting faces which form acute angles, a fusible element is secured to those con ductors at right angles to those confronting faces, and portions of those conductors are overlaid by layers of arc extinguishing material; and any are which forms as that fusible element fuses will lengthen by shifting toward the tips of the sections adjacent the confronting faces of the conductors, and the layers of arc extinguishing material will quickly quench any are that may burn back through those tips.

This invention relates to improvements in protectors for electric circuits. More particularly, this invention relates to improvements in electric fuses.

It is, therefore, an object of the present invention to provide an improved electric fuse.

In the construction and operation of low amperage electric fuses, it is frequently desirable to use bridge structures to support the fusible elements of those electric fuses. Those bridge structures are important because they provide the required support for the fusible elements of those electric fuses, and because they prevent accidental engagement between those fusible elements and the interiors of the casings of those electric fuses. One particularly valuable low amperage electric fuse that uses a bridge structure to support the fusible element thereof is disclosed in McAlister Patent No. 2,576,405. Unfortunately, that electric fuse is relatively expensive to manufacture; and hence it would be desirable to provide a low amperage electric fuse that has a bridge structure to support the fusible element thereof but that is relatively inexpensive to manufacture. The present invention provides such an electric ,fuse; and it is, therefore, an object of the present invention to provide an electric fuse which has a bridge structure to support the fusible element thereof but'which is relatively inexpensive to manufacture.

' The bridge structure of a preferred embodiment of electric fuse provided by the present invention is made from a piece of insulating material that has a thin, metallic layer intimately adhering to it; and portions of that thin, metallic layer are then covered with an acid-resistant ink. That acid-resisting ink is placed on spaced areas of the thin, metallic layer of the bridge structure to define two inkcovered, spaced, elongated, generally-aligned, metallic portions. The bridge structure is then immersed in an acid solution; and those portions of the thin, metallic layer of that bridge structure which are not covered by the acidresistant ink will dissolve in that solution. The inkcovered, spaced areas on the thin, metallic layer, that define the two spaced, elongated, generally-aligned metallic portions, will be substantially unaffected by the acid solution; and they will subsequently have the acid-resisting fink removed from them to provide two spaced, elon gated, generally-aligned conductors on the bridge'struc ture. An opening will then be punched through the bridge 3,348,007 Patented Oct. 17, 1967 ice spaced, elongated, generally-aligned, conductors; and the fusible element will be disposed in engagement with those confronting faces and in register with that opening to enable a substantial portion of the length of that fusible element to be spaced away from all portions of that bridge structure.

The two spaced, elongated, generally-aligned conductors on the bridge structure will have the confronting faces thereof inclined relative to the elongated axis of that bridge structure; and those confronting faces will coact with adjacent edges of those spaced, elongated, generallyaligned conductors to define sections of those conductors that from acute angles. The fusible element will preferably be disposed so it is at right angles to each of the the confronting faces of the spaced, elongated, generally- :aligned conductors; and hence that fusible element can have an effective length which is no longer than the minimum distance between the confronting faces of those spaced, elongated, generally-aligned conductors. This is desirable because a short fusible element adds less resist ance to an electric circuit than does a long fusible element of the same material and cross section. Although the electrio fuse provided by the present invention has a short fusible element, that electric fuse is able to safely interrupt circuits across the appreciable voltages are developed; because the sections of the two spaced, elongated, generally-aligned conductors which are formed as acute angles will tend to lengthenand thus tend to interruptany arc that forms as that fusible element fuses. Specifically, as such an are forms, the ends of that are will tend to shift towards the tips of those sections of the spaced, elongated, generally-aligned conductors on the birdge structure which are formed as acute angles, because arcs can sustain them selves more readily between points than between surfacessurfaces tending to cool and quench the arcs. As the ends of the arc shift toward the tips of those sections of the spaced, elongated, generally-aligned conductors, the overall length of the arc will increase; and that increase will tend to extinguish that arc. It is, therefore, an object of the present invention to provide an electric fuse which has a bridge structure with spaced, elongated, generally-aligned conductors that have sections thereof which form acute angles, which has an opening in that bridge structure intermediate the confronting-faces of those conductors, and which has the fusible element thereof disposed generally at right angles to those confronting faces.

The spaced, elongated, generally-aligned conductors on the bridge structure of the electric fuse of the present invention are overlain, in part, by a material which is resistant to arcing. Preferably, only the adjacent ends, of the spaced, elongated, generally-aligned conductors on the bridge structure, to which the ends of the fusible element are soldered and the other ends, of those conductors which are soldered to the terminals of the electric fuse, are not covered by the arc-resistant material. Where that is done, the electric fuse will be able to safely open circuits across which surprisingly high current short circuits are developed. It is, therefore, an object of the present invention described but it is to be understood that the drawing structure intermediate the confronting faces of the two and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will de defined by the appended claims.

in the drawing, 1 is a plan view of a piece of insulation that has a thin, metallic layer intimately adhering to it and that is used in making the bridge structure of one preferred embodiment of electrical fuse that is made in accordance with the principles and teachings of the present invention,

FIG. 2 is a front elevational view of the structure shown in FIG. 1,

FIG. 3 is a plan view of the structure of FIG. 1 after acid-resisting ink has been used to cover two spaced, elongated generally-aligned areas on the metallic layer of the structure of FIG. 1,

FIG. 4 is a plan view of the structure of FIG. 1 after an acid solution has dissolved those portions of the metallic layer which were not overlain by the acid-resistant ink and after that acid-resistant ink has been removed,

FIG. 5 is a plan view of the structure of FIG. 1 after an opening has been formed therein intermediate the confronting faces of the two spaced, elongated, generallyaligned conductors thereon,

FIG. 6 is a plan view of the structure of FIG. 1 after arc-resistant material has been applied to the central areas of the spaced, elongated, generally-aligned conductors on that structure,

FIG. 7 is a plan view of the structure of FIG. 1 after a fusible element has been soldered to the confronting faces of the spaced, elongated, generally-aligned conductors on that structure,

FIG. 8 is a plan view, on a larger scale, of a portion of the structure of FIG. 1, and it indicates, by dotted lines, the position which an arc would assume after the fusible element fuses, and

FIG. 9 is a longitudinal section through an electric fuse in which the structure of FIG. 7 is incorporated.

Referring to the drawing in detail, the numeral 20 denotes a piece of insulation to which a thin, metallic layer 22 intimately adheres. In one preferred embodiment of the present invention, the layer 22 consists of copper or a cuprous alloy. That layer can be made as thin as desired, and the thickness of that layer can be controlled with great precision. As indicated particularly by FIG. 1, the piece 20 of insulation is wider adjacent the center thereof than it is adjacent the ends thereof.

The numeral 24 and the numeral 26 denote acid-resistant ink applied to the metallic layer 22; and, as shown particularly by FIG. 3, the areas of that metallic layer which underlie that ink are spaced, are elongated, and are generally aligned. After the application of the ink 24 and 26 to the metallic layer 22, the piece 20 of insulation with its intimately adherent layer is immersed in an acid solution; and that acid solution will remove all portions of the metallic layer 22 which are not overlain by the ink 24 and 26. After the piece 20 of insulation has been removed from the acid solution and has been rinsed to remove all of that acid solution, the acid resistant ink 24 and 26 will be removed.

As the piece 20 of insulation is removed from the acid solution, the size and configuration of the conductor 28, which is overlain by the acid-resistant ink 24, will conform almost exactly to the size and configuration of the area defined by the ink 24. Similarly, the size and configuration of the conductor 30, which is overlain by the acid-resistant ink 26, will conform almost exactly to the size and configuration of the area defined by the ink 26. As a result, the conductor 28 and the conductor 30 will be elongated and generally-aligned but will have the confronting faces thereof spaced apart. Those confronting faces will be inclined to the long axis of the piece 20 of insulation, and they will be offset from that longitudinal axis, as shown particularly by FIG. 4. Also, those confronting faces will coact with adjacent edges of those conductors to define sections of those conductors which form acute angles.

An opening 32 is formed in the piece 20 of insulation intermediate the confronting faces of the conductors 28 and 30. That opening is shown as being circular in plan, but that opening could have any desired configuration. The opening 32 is preferably spaced a short distance from each of the confronting faces of the conductors 28 and 30, so that opening can be formed by a punching operation without affecting the precisely-etched confronting faces of the conductors 28- and 30.

The central portions of the conductors 28 and 30 will be covered, respectively, by arc-resistant material 34 and 36. While different arc-resistant materials could be used, an epoxy polyester has been found to be very useful. Such a material is stable until the fusible element fuses, and then that material will rapidly evolve a gas which will tend to extinguish any are that may form as that fusible element fuses. In addition, that material has a high dielectric strength; and it will strongly resist any effort of the current to track across it.

The piece 20 of insulation with the spaced, elongated, generally-aligned conductors 28 and 30 thereon will act as a bridge structure for a low amperage electric fuse. That bridge structure will be sturdy and rugged, and it will fully support the fusible element that will be soldered to it. Also, that bridge structure will keep that fusible element from engaging the interior of the easing of that fuse.

The outer ends of the conductors 28 and 30 are left bare to facilitate the soldering of those conductors to the terminals of the electric fuse in which the bridge structure is to be incorporated. The confronting faces of those conductors also are left bare to facilitate the soldering of a fusible element 38 to those conductors. Small masses of solder 40 and 42 are used to secure the fusible element 38 to the confronting faces of the conductors 2'8 and 30. The arc-resistant material 34 and 36 will adhere so intimately to the conductors 2'8 and 30 that it will keep the solder 40 and 42 from flowing onto those portions of those con-ductors which are overlain by that material. This is desirable, because it will enable the soldering of the fusible element 38 to the conductors 28 and 30 to be done automatically by a machine rather than by a workman.

It will be noted that the fusible element 38 is disposed at an angle to the long axis of the bridge structure and is disposed at right angles to the confronting faces of the conductors 28 and 30. Further, it will be noted that the fusible element 38 is secured to the confronting faces of the conductors 28 and 30 at points intermediate the ends of those confronting faces. Because that fusible element is disposed at right angles to the confronting faces of the conductors 28 and 30, and because the ends of that fusible element are secured to the confronting faces of those conductors at points intermediate the ends of those confronting faces, the elfective length of the fusible element 38 is less than the distance between the tips of those sections of those conductors which form acute angles.

The wide portion of the piece 20 of insulation is dimensioned to telescope freely within the tubular casing 44 of the'electric fuse shown in FIG. 9; and the overall length of that piece of insulation is substantially the same as the overall length of that casing. Solder 50 is used to bond the outer end of the conductor 28 to the ferrule-like terminal 46 at the left-hand end of that electric fuse, while solder 52 is used to bond the outer end of the conductor 30 to the ferrule-like terminal 48 at the right-hand end of that electric fuse. Because the wide portion of the piece 20 of insulation is just slightly smaller than the inner diameter of the casing 44 of the electric fuse, that wide-portion automatically spaces the conductors 28 and 30 and the fusible element 38 away from the interior of that casing. This is desirable, because any contact between those conductors and that casing, or any contact between that fusible element and that casing, could enable the heat-absorbing capability of G that casing-to seriously affect the rating of the electric fuse.'

The electric fuse shown in FIG. 9 will carry its rated current continuously; and it will carry low overloads for short periods of time. However, before any overload could becontinued long enough to constitute a potential threat to the circuit, protected by that electric fuse, the fusible element 38' would fuse and open that circuit. If an are formed as that fuse opened, that are would tend to shift in the clockwise direction from the axis of the fusible element 38 toward the dotted line in FIG. 8 which extends between the tips of those sections of the conductors 28 and 30 which form acute angles. As that are rotates in the clockwise direction, the length thereof will increase; and the tendency of that are to extinguish itself also will increase. As a result, in many cases, the arc will extinguish itself with very little volatilization of the confronting faces of the conductors 28 and 30'.

In the event an arc is established that tends to volatilize appreciable portions of the confronting faces of the conductors 28 and 30, the arc-resistant material 34 and 36 will promptly and rapidly evolve a gas which will help extinguish that are. As a result, the electric fuse provided by the present invention can be used to protect circuits across which appreciable voltages are developed, and can safely interrupt those circuits even when extremely heavy short circuits occur. For example, an electric fuse which has an overall length as short as one inch, which has a casing with an outer diameter of less than one quarter of an inch, which has a fusible element with an effective length of about one-eighth of an inch, and which is rated at one-eighth of an ampere can safely interrupt ten thousand amperes at two hundred and fifty volts. Without the arc-resistant material 34 and 36, the casing of that fuse might crack or that fuse might fire cotton surrounding it when that fuse was called upon to interrupt as few as one hundred amperes at two hundred and fifty volts.

Whereas the drawing and accompanying description have shown and described a preferred embodiment of the present invention it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof.

What I claim is:

1. An electric fuse which comprises:

a housing,

an insulating support disposed within said housing,

a conductor on said support,

a second conductor on said support,

said conductors being spaced apart but having confronting faces,

said confronting faces being generally straight and being inclined relative to the axis of said housing, said conductors having sections adjacent said confrontin g faces that form acute angles, and

a fusible element secured to said confronting faces of said conductors,

said sections of said conductors having the tips thereof,

which are adjacent said confronting faces of said conductors, spaced inwardly fromand completely out of engagement with the inner surface of said housing, whereby said tips will not be cooled by said housing,

said fusible element having the ends thereof spaced away from said tips of said sections adjacent said confronting faces of said conductors,

said conductors tending to cause the ends of any arc,

that may form as said fusible element fuses, to shift toward said tips of said sections adjacent said confronting faces of said conductors, and thereby increase the overall length of said are to effect prompt extinction of said arc.

2. An electric fuse as claimed in claim 1 wherein said conductors define a plane, and wherein said insulating support has an opening therein which extends through said support and which is generally normal to said plane defined by said conductors, said opening being intermediate said confronting faces of said conductors to space portions of said fusible element wholly away from said insulating support.

3. An electric fuse as claimed in claim 1 wherein said fusible element is disposed at right angles to said confronting faces of said conductors to limit the effective length of said fusible element to a minimum and wherein the ends of the fusible portion of said fusible element are closer to said tips than they are to the other ends of said confronting faces.

4. An electric fuse as claimed in claim 1 wherein said confronting faces of said conductors are inclined at an angle to the axis of said insulating support and wherein said confronting faces are substantially parallel.

5. An electric fuse as claimed in claim 1 wherein layers of arc-resistant material overlie portions of said conductors to help extinguish any are that may form as said fusible element fuses, wherein said layers of arc-resistant material are disposed between said tips and the outer ends of said conductors, and wherein said layers of arc-resistant material are spaced outwardly from said tips and from the fusible portion of said fusible element.

6. An electric fuse as claimed in claim 1 wherein layers of arc-resistant material overlie portions of said conductors, said layers of arc-resistant material being spaced outwardly of the fusible portion of said fusible element but responding to the fusing of said fusible element to promptly and rapidly evolve a gas that can help extinguish any are that may form as said fusible element fuses.

7. An electric fuse as claimed in claim 1 wherein layers of arc-resistant material overlie portions of said conductors to help extinguish any are that may form as said fusible element fuses, said portions of said conductors being adjacent to, but being spaced outwardly of, said confronting faces of said conductors and also being spaced outwardly of the fusible portion of said fusible element.

8. An electric fuse which comprises:

an insulating support,

a conductor on said support,

a second conductor on said support,

said conductors being spaced apart but having confronting faces,

a fusible element secured to said confronting faces of said conductors, and

layers of arc-resistant material overlying portions of said conductors to help extinguish any are that may form as said fusible element fuses, said layers of arc-resistant material being spaced outwardly of said confronting faces and being spaced outwardly of the fusible portion of said fusible element.

9. An electric fuse as claimed in claim 8 wherein said arc-resistant material is an epoxy polyester.

10. An electric fuse as claimed in claim 8 wherein said layers of arc-resistant material keep solder from flowing into engagement with said portions of said conductors.

11. An electric fuse which comprises:

a housing,

an insulating support disposed within said housing,

a conductor on said support,

a second conductor on said support,

said conductors being spaced apart but having confronting faces,

said confronting faces being generally straight and being inclined relative to the axis of said housing, said confronting faces of said conductors having tips,

and

a fusible element secured to said confronting faces of said conductors,

said tips of said confronting faces of said conductors 7 8 i being spaced inwardly from and completely out of and being spaced outwardly of the fusible portion of said engagement with the inner surface of said housing, fusible element.

whereby said tips will not be cooled by said housing,

said fusible element having the ends thereof spaced References cued away from said tips on said confronting faces of 5 UNITED STATES PATENTS said conductors, 2,502,992 4/ 1950 Rawlins et al 200120 X said conductors tending to cause the ends of any are, 2,576,405 11/1951 M Ali ter 200135 X that may form as said fusible element fuses, to shift 2,964,604 12/1960 J acob et; 1 200--120 toward said tips on said confronting faces of said 0011- 2,941,059 6/ 1960 Sims et al 200-135 t ductors, and thereby increase the overall length of 10 said arc. FOREIGN PATENTS 12. An electric fuse as claimed in claim 11 wherein 499,816 1/1939 Great Britain layers of arc-resistant material overlie portions of said conductors to help extinguish any are that may form as NARD A GILHEANY, Primary Examiner, said fusible element fuses, said layers of arc-resistant 15 material being spaced outwardly of said confronting faces GILSON Asslsmnt E xammer' 

8. AN ELECTIRC FUSE WHICH COMPRISES: AN INSULATING SUPPORT, A CONDUCTOR ON SAID SUPPORT, A SECOND CONDUCTOR ON SAID SUPPORT, SAID CONDUCTORS BEING SPACED APART BUT HAVING CONFRONTING FACES, A FUSIBLE ELEMENT SECURED TO SAID CONFRONTING FACES OF SAID CONDUCTORS, AND LAYERS OF ARC-RESISTANT MATERIAL OVERLYING PORTIONS OF SAID CONDUCTORS TO HELP EXTINGUISH ANY ARC THAT MAY FORM AS SAID FUSIBLE ELEMENT FUSES, SAID LAYERS OF ARC-RESISTANT MATERIAL BEING SPACED OUTWARDLY OF SAID CONFRONTING FACES AND BEING SPACED OUTWARDLY OF THE FUSIBLE PORTION OF SAID FUSIBLE ELEMENT. 